In general the present invention relates to telephone station equipment, and more particularly to multi-line telephone systems having no central control unit.
Telephone installations in which calls can be placed or received at any one of a number of telephone sets, and over any one of a number of available telephone lines, are well known and are called key telephone systems (KTS). Installations requiring five or more lines typically include a central control unit, which must be set up at a location remote from the telephone sets. It is generally not economical for businesses which require less than five lines to install and operate a key telephone system having a central control unit. One system which does not require a central control unit is disclosed by Rasmussen in U.S. Pat. Nos. 4,132,860 and 4,133,985.
An important feature of a key telephone system is the ability to place a telephone line on hold from any one of the telephone sets, and also to release the hold from any of the telephones within the system. The hold condition is achieved by connecting a line terminating impedance across the line that is to be held. This terminating impedance simulates an off-hook condition which causes the central office to maintain the connection with the remote telephone. In U.S. Pat. No. 4,132,860, Rasmussen teaches the application of a low frequency (0.5 Hz) square wave across the telephone line to indicate that the line is in a hold station. This fluctating signal, which is produced by a zener diode circuit, is transmitted to visual indicator lights at each remote telephone.
The hold control circuit of Rasmussen presents several disadvantages. First, the low frequency square wave which indicates a hold condition may also produce an audible periodic clicking sound which may be objectionable to the party on hold. Second, for proper operation on long (high resistance) subscriber loops, a telephone set must either provide a d.c. resistance of 330 ohms maximum, or maintain a minimum loop current of 26 mA for loop voltages above8.58 volts. The preferred breakdown voltage for the two zener diodes in Rasmussen's circuit is 20 volts. Thus, a 48 volt system voltage (also called exchange battery voltage, regardless of actual EMF source) battery in a 1500 ohm subscriber loop would produce a d.c. loop current of 18.7 mA in the Rasmussen circuit, which is too low to ensure proper central office line supervision. Finally, telephone lines in the United States should generally be terminated in an a.c. impedance of 600 ohms. The a.c. impedance range is constrained by the allowable return loss in the voice band of 7 dB, which implies a minimum a.c. impedance of 230 ohms. The dynamic resistance of zener diode varies depending on manufacture, but values of Rz=5 ohms are typical. Thus, the circuit disclosed by Rasmussen would have a typical a.c. impedance of 10 ohms, which is unacceptably low, in that it would result in reflection of signals in the voice band.